1. Introduction
An I/Vconvertor is intended to convert the current from a DAC (Digital Analog Convertor) into an analog voltage and to filter this signal to keep out the alias frequencies. Often these functions are separated into two stages: one for the conversion, and another for the low pass filtering. I will call them: 'the I/VC' and 'the lpfilter'.
The current from the DAC (often 2.5 mA) is far from clean. Outside the audio frequency domain the aliases and restsignals of digital origin with a wide frequency spectrum do exist.
The input impedance of the I/VC should be constant and low (< 5 ohm) over the whole frequency range in question. 

2. The I/VC
What about the high frequency components in the DACcurrent? Cannot they be filtered out before entering high open loop gain stages? Cannot high open loop gain stages be avoided at all? Because of the low distortion figures and a reasonable low output impedance, at least the lpfilter does need feedback. Because of the prescribed low input impedance of the I/VC, it must start with an emitter or sourceinput stage if no use is made of 'the virtual groundcircuit' with an op amp.
2.1 I/VC with discrete bipolars
The discrete I/VCcircuit with bipolar transistors (at the right) performs best if the second harmonics have been cancelled with the proper value of R1 and R2. The third harmonic however is still .04 %. Plots of the I/VC with discrete bipolar transistors 

2.2 I/VC with a FET
The discrete I/VCcircuit with ten times the FET J310 in parallel in GGS performs very well. The frequency response is excellent and the distortion is low. The circuit shown at the right is only for simula tion purposes. Of course in practice the DClevel of the input should be 0 volt. This will be realised by biasing the gate. Plots of the I/VC with FET 

2.3 I/VC with an Op Amp
If we stick to low distortion, we must resort to op amps. The question is: which op amp will perform best. The circuit diagram of an opampI/VC is rather simple. 

2.3.1 I/VC with an OPA648
The wide band, emitterinput current feedback, single op amp OPA648 is unity gain stable, with a 
Plots of the I/VC with an OPA648 
2.3.2 I/VC with an OPA134
The ultra low distortion, FETinput, op amp OPA134 is 
Plots of the I/VC with an OPA134 
2.3.3 I/VC with an LT1028
The ultra low noise, single op amp LT1028 is unity gain stable, with a 
Plots of the I/VC with an LT1028 
2.3.4 I/VC with an AD826
The dual op amp AD826 is unity gain stable, with a 
Plots of the I/VC with an AD826 
2.4 Final remarks to Op Amp I/VC's
If an op amp lacks bandwidth so that the input impedance increases at high frequencies, could this be compensated by shunting de I/VCentry with a capacitor? The answer is simply: NO! The input impe dance of a virtual ground becomes inductive at high frequencies, so a capacitive shunt makes things worse. The circuit becomes fluid. Even oscillation cannot be excluded.


3. The lpfilter
The I/VC's presented here have a low pass filter of 6 dB/oct which is undeniable inadequate for the total performance. 

3.1 LPfilter with OPA648's
This op amp performs well with C8 = 2.2 nF but there is still some anomaly in the frequency characteristic near 120 MHz. 
Plots of a lpfilter with OPA648 
3.2 LPfilter with LT1028's
This op amp performs well with C8 = 2.2 nF but there is still some anomaly in the frequency characteristic near 30 MHz (at 160 dB!). 
Plots of the lpfilter with LT1028 
3.3 LPfilter with AD826's
The distortion is no problem but the slope in the frequency characteristic bends back upwards at 20 MHz even with 
Plots of a lpfilter with AD628 
3.4 LPfilter with an OPA2134
Former experiences appointed this op amp as sounding very well. Moreover Henk ten Pierick measured it as the best op amp in high impedance applications. The distortion is out of the question but the lack of bandwidth also here plays a role: at about 20 MHz the slope bends back upwards and downwards again. With C8 = 2.2 nF this 'swing' is 120 dB down. 
Plots of the lpfilter with an OPA2134 
3.2 LPfilter with LME49710's
This very special ultra low distortion (0.00003%), low noise single op amp LME49710 is unity gain stable, with a 
Plots of the lpfilter with LME49710

4 Resuming
Up till now, the LME49710 has not been evaluated. 

5 The Mature CircuitA rather long time later I investigated an I/Vconverter for the PCM1792/PCM1795 which both have a balanced current output. Beforehand the unbalanced one had been investigated again, which means that the combination of the I/VC and the lpfilter was ameliorated. The results are in the small document: The Mature I/Vconverter. 
6 Conclusions
For the circuits investigated up till now an I/VC with the LT1028 pointed out to be a good choice. 

April  2013 Herbert Rutgers. 